Cable including optical fibers enclosed in a sheath with longitudinal reinforcing elements

ABSTRACT

A cable including optical fibers ( 1 ) surrounded by a sheath ( 2 ) comprising at least one layer in which at least two elongate reinforcement members ( 4 ) extend, the cross section of the sheath having a substantially oval outline ( 5 ) presenting a minor axis ( 5.1 ), and the reinforcement members being disposed symmetrically on either side of the minor axis.

This application is the national phase under 35 U.S.C. §371 of PTCInternational Application No. PCT/FR99/01118 which has an Internationalfiling date of May 11, 1999, which designated the United States ofAmerica.

The present invention relates to a cable including optical fiberssurrounded by a sheath having longitudinal reinforcement.

BACKGROUND OF THE INVENTION

In the cable industry, it is well known that optical fibers must not besubjected to mechanical stress if their transmission performance ortheir lifespan are not to be degraded.

Optical fibers are therefore disposed in a sheath which protects themmechanically against traction, longitudinal compression, and crushingforces.

Furthermore, in order to limit stress on the optical fibers both whilethe cables are laid and during temperature variations which cause thematerials constituting the sheath to expand or retract, it is known touse reinforcement members extending in a layer of the sheath of thecable.

It is found that the resistance to crushing depends mainly on thethickness of the material forming the sheath. A cable-crushing forceapplied to the sheath over the reinforcement members is not absorbedsufficiently by the sheath and is therefore transmitted to thereinforcement members which run the risk, under said force, of causingthe sheath to tear. This results in a risk of damage to the opticalfibers.

Document FR-A-2 745 642 proposes an optical fiber cable having a sheaththat includes four diametrally opposite projecting ribs extending alongthe length of the cable. Such a cable has good crushing strength.However, it poses sealing problems both when connected to a connectionbox and when laid in a conduit by using a flow of compressed air.Furthermore, as a result of its structure, such a cable is difficult towind evenly onto a reel and slides with considerable friction in acylindrical conduit.

Document DE-A-26 35 917 discloses a cable which includes optical fiberssurrounded by a sheath whose cross-section has an oval outlinepresenting a minor axis. The sheath comprises a layer in which elongatereinforcement members extend, the reinforcement members being disposedon either side of the minor axis. The cable thus has greater crushingstrength along the direction of the minor axis and, when the cable restson a support, its oval outline causes the cable naturally to take up anorientation such that the minor axis extends perpendicularly to thesupport, i.e. in the direction in which crushing forces usually act.However, that cable is markedly oval in shape which makes sealingdifficult at connection boxes. The oval shape also enables the cable tobe wound evenly onto a reel and to slide with little friction in acylindrical conduit.

OBJECTS AND SUMMARY OF THE INVENTION

An object of the invention is to propose an optical fiber cable whichcan withstand a significant amount of crushing, without affecting itsbending properties or its suitability for sealed mounting at its end.

In order to achieve this object, the invention provides a cableincluding optical fibers surrounded by a sheath whose cross-section hasa substantially oval outline presenting a minor axis, the sheathcomprising at least one layer in which at least two elongatereinforcement members extend, the reinforcement members being disposedsymmetrically on either side of the minor axis, the ovalization of theoutline lying in the range 1% to 10%.

The ovalization is thus sufficient for the cable naturally to take up aposition presenting greatest crushing strength in the appropriatedirection when it rests on a support. Furthermore, in this position, therelative flexibility of the cable in various directions is such as tominimize the risk of the cable undulating while it is being pushed intoplace in a conduit. In addition, the cable ends are generally mounted incylindrical endpieces with clearance such that the above-mentionedovalization of the outline of the cable does not affect the possibilityof providing a sealing of the inlet of connection or splice boxes.

BRIEF DESCRIPTION OF THE DRAWINGS

Other characteristics and advantages of the invention will appear onreading the following description of a particular non-limitingembodiment of the invention.

It is made with reference to the sole accompanying FIGURE which is across-section view of the cable of the invention.

MORE DETAILED DESCRIPTION

With reference to the FIGURE, the cable of the invention includes abundle of optical fibers 1 whose structure is not shown in detail, butwhich can, for example, be made in the form of one or more groups ofoptical fibers held together by a holding sheath, as described indocument FR-A-2 665 266.

The bundle of optical fibers is surrounded by an outer sheath 2, in thiscase made of a single layer, having embedded therein two groups 3, eachconstituted by two reinforcement members 4. The reinforcement membersare made of conventional materials such as wires, or strands of glass oraramid fiber stiffened by a resin having a high traction modulus and alow thermal expansion coefficient.

In the invention, the sheath at rest has a cross-section with an outlinethat is substantially oval in shape, as shown in the FIGURE. The outline5 shown presents a minor axis of symmetry 5.1 (vertical in the FIGURE)and a major axis of symmetry 5.2 (horizontal in the FIGURE), but theoval shape of the invention covers any curvilinear-shaped outline whosecurvature is essentially turned towards the inside of the outlinewithout being re-entrant, and has different dimensions along twosubstantially perpendicular directions, the directions being called axesalthough they are not necessarily axes of symmetry. In this case, thesheath is made by extrusion by means of a die having an orifice of ovaloutline, but other embodiments could be used.

The ovalization of the outline 5 lies in the range 1% to 10%.Ovalization is expressed as the ratio of twice the difference betweenthe long dimension and the short dimension of the outline (respectivelymeasured along the axes 5.2 and 5.1) over the sum of the long dimensionplus the small dimension of the outline.

The groups 3 of reinforcement members 4 extend symmetrically on eitherside of the minor axis of symmetry 5.1.

It will be understood that when the cable of the invention is laid on asupport 6 it is naturally oriented by its oval outline 5 so that theminor axis of symmetry 5.1 extends substantially perpendicularly to thesupport 6. Thus, if an operator walks on the cable, the crushing forceis applied along the direction of the axis of symmetry 5.1, i.e. thedirection in which the cable has the best crushing strength.Furthermore, this spontaneous orientation of the cable has the advantageof making it easier to locate the reinforcement members in the sheath,so that it is easy to make an incision in the sheath without damagingthe reinforcement members in order to access the optical fibers.

It should be noted that the ovalization values mentioned are sufficientto favor an orientation of the cable on the support, while ensuring thatit is still easy to seal the connection boxes to which the cable isconnected.

Naturally, the invention is not limited to the embodiment described andvariations can be applied thereto without going beyond the ambit of theinvention as defined by the claim.

In particular, although the cross section of the outer sheath has beenshown in the FIGURE as having a circular inside contour, its insidecontour could be oval in shape. Furthermore, the sheath could be amultilayer sheath.

Although the reinforcement members are described as being formed of twogroups of two reinforcement members each, a cable could be madecomprising only two reinforcement members disposed symmetrically aboutthe minor axis of symmetry, or comprising two groups each having morethan two reinforcement members.

What is claimed is:
 1. A cable including optical fibers (1) surroundedby a sheath (2) having a cross-section which has a substantially ovaloutline (5) presenting a minor axis (5.1) and a major axis (5.2), thesheath comprising at least one layer in which at least two elongatereinforcement members (4) extend, the reinforcement members beingdisposed symmetrically on either side of the minor axis, wherein saidoutline (5) has no re-entrant portion and has an ovalization, that is aratio of twice the difference between a longest dimension taken alongsaid major axis and a shortest dimension taken along said minor axis ofthe outline over a sum of the longest dimension plus the shortestdimension of the outline (5), which lies in the range of from 1% to 10%.